Admittedly, I have many, many backpacks, and I often get more looking for an ideal backpack for the occasion. I got the XL (30L) version of this backpack.
SMALL BACKPACKS (< 20L)Suitable for everyday carry, tech carry, day travel, tablets… While a capacity of <20L is considered a compact size for a backpack, don't let that fool you. If the backpack is built well and is fully utilized, you should still be able to fit lots of essentials into it.
Whichever backpack you choose, it needs to match your torso length. To determine your size, reach behind your neck, bend your head forward, and find the C7 vertebra: It's the bone that sticks out the most on your upper spine. Next, locate the iliac crest. It's the top of the hip shelf on the sides of your hips.
In general, a backpack is considered a personal item if it fits under the seat in front of you. This means that the backpack must be no larger than 22 inches in length, 14 inches in width, and 9 inches in height. If the backpack is larger than this, it will be considered luggage and must be checked.
However, a good rule of thumb is to choose a backpack that can comfortably accommodate their textbooks, notebooks, laptop or tablet, and other essential items. A backpack with a capacity of 20-30 liters is usually a good choice for most high school students.
It is going to be on your back carried through train stations, city streets, and airports. It is important to have a pack that is big enough to carry everything, but not too big to be a major burden. I recommend getting a backpacking style pack that is between 30-45L.
MEDIUM BACKPACKS (20 – 35L)Suitable for school, camera, work, gym, short travel, cabin luggage … This capacity range is considered a medium-size backpack and is probably the most popular bag capacity out there.
As a general rule of thumb, backpacks under 20L are allowed as free carry on baggage. Most airlines will charge extra to take larger backpacks on as carry on luggage. Can a 60L backpack be a carry on? Yes, a 60L backpack can be taken as a carry on.